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  <section id="shec">
<h1>SHEC 纠删码插件<a class="headerlink" href="#shec" title="Permalink to this heading"></a></h1>
<p><em>shec</em> 插件封装了 <a class="reference external" href="http://tracker.ceph.com/projects/ceph/wiki/Shingled_Erasure_Code_(SHEC)">multiple SHEC</a>
库。与 Reed Solomon 编码相比，它能使 Ceph 更高效地恢复数据。</p>
<section id="id1">
<h2>创建 SHEC 配置<a class="headerlink" href="#id1" title="Permalink to this heading"></a></h2>
<p>要新建 <em>shec</em> 纠删码配置：</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><style type="text/css">
span.prompt1:before {
  content: "$ ";
}
</style><span class="prompt1">ceph<span class="w"> </span>osd<span class="w"> </span>erasure-code-profile<span class="w"> </span><span class="nb">set</span><span class="w"> </span><span class="o">{</span>name<span class="o">}</span><span class="w"> </span><span class="se">\</span>
<span class="w">     </span><span class="nv">plugin</span><span class="o">=</span>shec<span class="w"> </span><span class="se">\</span>
<span class="w">     </span><span class="o">[</span><span class="nv">k</span><span class="o">={</span>data-chunks<span class="o">}]</span><span class="w"> </span><span class="se">\</span>
<span class="w">     </span><span class="o">[</span><span class="nv">m</span><span class="o">={</span>coding-chunks<span class="o">}]</span><span class="w"> </span><span class="se">\</span>
<span class="w">     </span><span class="o">[</span><span class="nv">c</span><span class="o">={</span>durability-estimator<span class="o">}]</span><span class="w"> </span><span class="se">\</span>
<span class="w">     </span><span class="o">[</span>crush-root<span class="o">={</span>root<span class="o">}]</span><span class="w"> </span><span class="se">\</span>
<span class="w">     </span><span class="o">[</span>crush-failure-domain<span class="o">={</span>bucket-type<span class="o">}]</span><span class="w"> </span><span class="se">\</span>
<span class="w">     </span><span class="o">[</span>crush-device-class<span class="o">={</span>device-class<span class="o">}]</span><span class="w"> </span><span class="se">\</span>
<span class="w">     </span><span class="o">[</span><span class="nv">directory</span><span class="o">={</span>directory<span class="o">}]</span><span class="w"> </span><span class="se">\</span>
<span class="w">     </span><span class="o">[</span>--force<span class="o">]</span></span>
</pre></div></div><p>其中：</p>
<p><code class="docutils literal notranslate"><span class="pre">k={data-chunks}</span></code></p>
<dl class="field-list simple">
<dt class="field-odd">描述<span class="colon">:</span></dt>
<dd class="field-odd"><p>各对象都被分割为 <strong>data-chunks</strong> 块，分别存储到不同的 OSD 上。</p>
</dd>
<dt class="field-even">类型<span class="colon">:</span></dt>
<dd class="field-even"><p>Integer</p>
</dd>
<dt class="field-odd">是否必需<span class="colon">:</span></dt>
<dd class="field-odd"><p>No.</p>
</dd>
<dt class="field-even">默认值<span class="colon">:</span></dt>
<dd class="field-even"><p>4</p>
</dd>
</dl>
<p><code class="docutils literal notranslate"><span class="pre">m={coding-chunks}</span></code></p>
<dl class="field-list simple">
<dt class="field-odd">描述<span class="colon">:</span></dt>
<dd class="field-odd"><p>为各对象计算出 <strong>coding-chunks</strong> 个编码块，并存储到不同 OSD 上。
<strong>coding-chunks</strong> 数值不一定等于宕机不丢数据所需的 OSD 数量。</p>
</dd>
<dt class="field-even">类型<span class="colon">:</span></dt>
<dd class="field-even"><p>Integer</p>
</dd>
<dt class="field-odd">是否必需<span class="colon">:</span></dt>
<dd class="field-odd"><p>No.</p>
</dd>
<dt class="field-even">默认值<span class="colon">:</span></dt>
<dd class="field-even"><p>3</p>
</dd>
</dl>
<p><code class="docutils literal notranslate"><span class="pre">c={durability-estimator}</span></code></p>
<dl class="field-list simple">
<dt class="field-odd">描述<span class="colon">:</span></dt>
<dd class="field-odd"><p>奇偶校验块数量，它们在各自的计算范围内包含了各数据块。此数值被用作 <strong>durability estimator</strong> （持久性估值）。例如，假设 c=2 ，就是说不丟数据的情况下可损失 2 个 OSD 。</p>
</dd>
<dt class="field-even">类型<span class="colon">:</span></dt>
<dd class="field-even"><p>Integer</p>
</dd>
<dt class="field-odd">是否必需<span class="colon">:</span></dt>
<dd class="field-odd"><p>No.</p>
</dd>
<dt class="field-even">默认值<span class="colon">:</span></dt>
<dd class="field-even"><p>2</p>
</dd>
</dl>
<p><code class="docutils literal notranslate"><span class="pre">crush-root={root}</span></code></p>
<dl class="field-list simple">
<dt class="field-odd">描述<span class="colon">:</span></dt>
<dd class="field-odd"><p>CRUSH 规则第一步所用的 crush 桶名字。例如 <strong>step take default</strong> 。</p>
</dd>
<dt class="field-even">类型<span class="colon">:</span></dt>
<dd class="field-even"><p>String</p>
</dd>
<dt class="field-odd">是否必需<span class="colon">:</span></dt>
<dd class="field-odd"><p>No.</p>
</dd>
<dt class="field-even">默认值<span class="colon">:</span></dt>
<dd class="field-even"><p>default</p>
</dd>
</dl>
<p><code class="docutils literal notranslate"><span class="pre">crush-failure-domain={bucket-type}</span></code></p>
<dl class="field-list simple">
<dt class="field-odd">描述<span class="colon">:</span></dt>
<dd class="field-odd"><p>要确保同一故障域内不能把任意两个块放进一个桶内。例如，如果故障域是 <strong>host</strong> ，那就不能把任意两个块放到同一主机。
它被用于创建 CRUSH 规则的一步，像 <strong>step chooseleaf host</strong> 。</p>
</dd>
<dt class="field-even">类型<span class="colon">:</span></dt>
<dd class="field-even"><p>String</p>
</dd>
<dt class="field-odd">是否必需<span class="colon">:</span></dt>
<dd class="field-odd"><p>No.</p>
</dd>
<dt class="field-even">默认值<span class="colon">:</span></dt>
<dd class="field-even"><p>host</p>
</dd>
</dl>
<p><code class="docutils literal notranslate"><span class="pre">crush-device-class={device-class}</span></code></p>
<dl class="field-list simple">
<dt class="field-odd">描述<span class="colon">:</span></dt>
<dd class="field-odd"><p>使归置限于指定的设备类（比如 <code class="docutils literal notranslate"><span class="pre">ssd</span></code> 或 <code class="docutils literal notranslate"><span class="pre">hdd</span></code> ）之内，在 CRUSH 图内用的是 crush 设备类的名字。</p>
</dd>
<dt class="field-even">类型<span class="colon">:</span></dt>
<dd class="field-even"><p>String</p>
</dd>
<dt class="field-odd">是否必需<span class="colon">:</span></dt>
<dd class="field-odd"><p>No.</p>
</dd>
</dl>
<p><code class="docutils literal notranslate"><span class="pre">directory={directory}</span></code></p>
<dl class="field-list simple">
<dt class="field-odd">描述<span class="colon">:</span></dt>
<dd class="field-odd"><p>设置纠删码插件所在目录（ <strong>directory</strong> ）。</p>
</dd>
<dt class="field-even">类型<span class="colon">:</span></dt>
<dd class="field-even"><p>String</p>
</dd>
<dt class="field-odd">是否必需<span class="colon">:</span></dt>
<dd class="field-odd"><p>No.</p>
</dd>
<dt class="field-even">默认值<span class="colon">:</span></dt>
<dd class="field-even"><p>/usr/lib/ceph/erasure-code</p>
</dd>
</dl>
<p><code class="docutils literal notranslate"><span class="pre">--force</span></code></p>
<dl class="field-list simple">
<dt class="field-odd">描述<span class="colon">:</span></dt>
<dd class="field-odd"><p>覆盖已存在的同名配置。</p>
</dd>
<dt class="field-even">类型<span class="colon">:</span></dt>
<dd class="field-even"><p>String</p>
</dd>
<dt class="field-odd">是否必需<span class="colon">:</span></dt>
<dd class="field-odd"><p>No.</p>
</dd>
</dl>
</section>
<section id="id2">
<h2>SHEC 布局概述<a class="headerlink" href="#id2" title="Permalink to this heading"></a></h2>
<section id="id3">
<h3>空间效率<a class="headerlink" href="#id3" title="Permalink to this heading"></a></h3>
<p>空间效率是数据块在所有对象内占的比率，并表示为 k/(k+m) 。
为提升空间效率，你可以增加 k 值或降低 m 值。</p>
<blockquote>
<div><p>SHEC(4,3,2)的空间效率 = <span class="math notranslate nohighlight">\(\frac{4}{4+3}\)</span> = 0.57
SHEC(5,3,2) 或 SHEC(4,2,2) 提升了 SHEC(4,3,2) 的空间效率</p>
</div></blockquote>
</section>
<section id="id4">
<h3>持久性<a class="headerlink" href="#id4" title="Permalink to this heading"></a></h3>
<p>SHEC 的第三个参数（ =c ）是一个持久性估值，它大致等于在不丢数据的前提下允许损失的 OSD 数量。</p>
<p><code class="docutils literal notranslate"><span class="pre">durability</span> <span class="pre">estimator</span> <span class="pre">of</span> <span class="pre">SHEC(4,3,2)</span> <span class="pre">=</span> <span class="pre">2</span></code></p>
</section>
<section id="id5">
<h3>恢复效率<a class="headerlink" href="#id5" title="Permalink to this heading"></a></h3>
<p>描述恢复效率的计算超出了本文档范畴，但是，提高 m 而不提高 c
确实能提高恢复效率。（然而，在这种情况下必须注意空间效率的牺牲）</p>
<p><code class="docutils literal notranslate"><span class="pre">SHEC(4,2,2)</span> <span class="pre">-&gt;</span> <span class="pre">SHEC(4,3,2)</span> <span class="pre">:</span> <span class="pre">实现了恢复效率的提升</span></code></p>
</section>
</section>
<section id="id6">
<h2>纠删码配置实例<a class="headerlink" href="#id6" title="Permalink to this heading"></a></h2>
<div class="highlight-default notranslate"><div class="highlight"><pre><span class="prompt1">ceph<span class="w"> </span>osd<span class="w"> </span>erasure-code-profile<span class="w"> </span><span class="nb">set</span><span class="w"> </span>SHECprofile<span class="w"> </span><span class="se">\</span>
<span class="w">    </span><span class="nv">plugin</span><span class="o">=</span>shec<span class="w"> </span><span class="se">\</span>
<span class="w">    </span><span class="nv">k</span><span class="o">=</span><span class="m">8</span><span class="w"> </span><span class="nv">m</span><span class="o">=</span><span class="m">4</span><span class="w"> </span><span class="nv">c</span><span class="o">=</span><span class="m">3</span><span class="w"> </span><span class="se">\</span>
<span class="w">    </span>crush-failure-domain<span class="o">=</span>host</span>
<span class="prompt1">ceph<span class="w"> </span>osd<span class="w"> </span>pool<span class="w"> </span>create<span class="w"> </span>shecpool<span class="w"> </span>erasure<span class="w"> </span>SHECprofile</span>
</pre></div></div></section>
</section>



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